Domain propagation arrangement



Sept. 15, 1970 A. H. BOB ECK 3,529,303

DOMAIN PROPAGATION ARRANGEMENT Filed Nov. 12, 1968 l3 I I "TINTERROGATESOUCE D 12 i in I PULSE SOURCE I' UTILIZ- PULSE 9R ATION SOURCEPROPAGATION CIRCUIT PULSE /I2 SOURCE CONTROL CIRCUIT F/GZ FIG. 4 2I 22 IF I I W FIG. 3 2' I I l M I I //\/l/EN7'OR 0 22 23 I By A. H BOBCKATTORNEY United States Patent O 3,529,303 DOMAIN PROPAGATION ARRANGEMENTAndrew H. Bobeck, Chatham, N.J., assignor to Bell TelephoneLaboratories, Incorporated, Murray Hill and Berkeley Heights, N.J., acorporation of New York Filed Nov. 12, 1968, Ser. No. 774,766 Int. Cl.G11c 11/14, 19/00 US. Cl. 340174 6 Claims ABSTRACT OF THE DISCLOSURE Anexternal means for supplying a bias field of a polarity to contractsingle wall domains employed to insure the stability of single walldomains in some orthoferrite sheets has been found to be unnecessary ifthe surface layers of the sheet are prepared so that those surfaces arepermanently magnetized normal to the sheet and exchangecoupled to thebody of the sheet.

FIELD OF THE INVENTION This invention relates to data processingarrangements and, more particularly, to such arrangements employingmagnetic media in which single wall domains can be propagated.

BACKGROUND OF THE INVENTION A single wall domain is a magnetic domainbounded by a domain wall which closes on itself and has a geometryindependent of the boundary of the sheet in which it is moved. Such adomain conveniently assumes the shape of a circle (viz., cylinder) inthe plane of the sheet and has a diameter determined primarily by thematerial parameters. The Bell System Technical Journal, vol. XLVI, No.8, October 1967, at pages 1901 et seq., describes the propagation ofsingle wall domains in a propagation medium such as a sheet of a rareearth orthoferrite.

One mode of operating with single wall domains is referred to as thebias mode because a bias field of a polarity to contract domains isgenerated in the material under normal operating conditions in order tomaintain constant the diameters of single wall domains therein. For asheet of orthoferrite, which is characterized by a preferred directionof magnetization essentially normal to the plane of the sheet, the biasfield also is essentially normal. If, for example, we adopt theconvention that a single wall domain has its magnetization in a positivedirection along an axis normal to the plane of the sheet, the domain canbe represented as an encircled plus sign and the bias field is taken tobe in a negative direction along that axis.

The bias field is usually generated by a coil encompassing the sheet, inwhich single wall domains are moved, and oriented in the plane of thatsheet. A suitable drive arrangement provides a current in the coil forgenerating the field in a controllable manner.

BRIEF DESCRIPTION OF THE INVENTION It has been found that the bias fieldcan be provided by preparing the surfaces of the sheet in which domainsare moved such that the surface layers are permanentl set magneticallyalong a first preferred direction of magnetization and exchange coupledto the body of that sheet. Bias mode operation with domains ofessentially constant diameter is thus provided in the absence of anexternal bias implementation. In one embodiment of this invention asheet of terbium orthoferrite which is bismuth doped is prepared withsurface layers permanently set by a magnetic field. Single wall domainsmoved in such a sheet are somewhat spherical in shape and have anessentially constant diameter.

3,529,303 Patented Sept. 15, 1970 BRIEF DESCRIPTION OF THE DRAWINGDETAILED DESCRIPTION FIG. 1 shows a domain propagation arrangement 10 inaccordance with this invention. The arrangement includes a magneticsheet 11 in which single wall domains are moved. Only a single channelfor domain propagation is shown. It is to be understood, however, thatthe channel is only representative and that others may be defined in asimilar manner either parallel to the one shown or at an angle theretoin accordance with the teaching of the abovementioned publication.

The channel is defined illustratively by a succession of conductorloops, l1, l2, l3 In, which are pulsed to generate consecutive localizedmagnetic fields (gradients) to attract a single wall domain from aninput to an associated output position. The conductor loops areconnected between a propagation pulse source 12 and ground to this end.

Single wall domains are introduced into a domain propagation channelfrom a source of domains illustratively comprising a region R ofpositive magnetization in accordance with the adopted convention. RegionR is encompassed by a conductor 13 which is connected between a DC.source S and ground. A hairpin conductor 14 is associated with source 13to separate a portion D thereof when pulsed. Conductor 14 is connectedbetween input pulse source 15 and ground. When source 15 pulsesconductor r14, portion D is separated from region R and becomes a singlewall domain for propagation. Conductor 13 oeprates to restore region Rto its normal shape when the pulse in conductor 14 terminates. If apulse is absent in a particular input time slot, no domain is generatedof course. Binary ones and zeros thus are represented by the presenceand absence of single wall domains respectively.

A domain pattern thus repersentative of information is moved along apropagation channel, by the propagation loops pulsed in a familiarthree-phase manner, towards an output position.

An output position is defined illustratively by a conductor 16 whichloops the rightmost (terminal) propagation loop (In) as viewed inFIG. 1. Conductor 16 is connected between an interrogate pulse source 17and ground and serves to collapse a domain present in the position socoupled. A conductor 18 also couples such terminal position. Conductor1-8 is connected between a utilization circuit 19 and ground. If adomain is present in the coupled terminal position when an interrogatepulse is applied to conductor 16, conductor 18 applies a pulse toutilization circuit 19. The interrogate pulse is usually appliedsynchronously with an input pulse and a selected propagation pulse andthe various circuits and sources are connected to a control circuit 20for appropriate activation and synchronization.

The various circuits and sources may be any such elements capable ofoperating in accordance with this invention.

Operation of a domain propagation device in the bias mode utilizes abias field normal to the plane of sheet 1 1. Such a field is normallyprovided by a current in a coil in the plane of sheet 11 occupying theposition of imaginary circle B. In accordance with this invention,domains can be moved in sheet 11 with essentially constant diameters inthe absence of such a bias field. The equivalent of the bias field inprovided by the structure of sheet 11.

A simple way to demonstrate the efi'ectiveness of a structure inaccordance with this invention in providing a bias equivalent field isto examine that structure under different magnetic conditions, expressthe differences in conditions in mathematical terms, and compare theresulting terms with an expression for a generated bias field. With thiscontext in mind, we can examine the structure of sheet 11 as shown in animaginary side view in FIG. 2. Sheet 11 may be thought of as includingthree separate layers which may in fact be a single body as is discussedfurther hereinafter. The three layers are designated 21, 22, and 23 inFIG. 2. Layers 21 and 23 have the property that the magnetizationtherein can be set permanently while that of layer 22 can be changed atspecified fields. Layers 21 and 23 are assumed to have magnetizationdirected upward as indicated by the upward directed arrows 24 and 25 inFIG. 2. Layer 22, on the other hand, is assumed to have itsmagnetization directed downward, as indicated by downward directed arrow26 in FIG. 2, in the single wall domain which is designated D as inFIG. 1. The remainder of layer 22 has its magnetization directed upwardas indicated by the upward directed arrows 27. It is clear that a domainwall exists between domain D and the remainder of layer 22 as well asbetween domain D and each of the surface layers 21 and 23. The domainwall is designated DW in FIG. 2.

FIG. 3 shows the magnetization in domain D and the remainder of layer 22reversed from that shown in FIG. 2. It is clear from FIG. 3 that nodomain wall exists between domain D and the magnetization of layers 21and 23.

Consider, in this context, the domain wall energy that can exist in theinterfaces of layers 21 and 22 and layers 22 and 23. It is clear that inFIG. 2 the total wall energy at these interfaces is where a is the wallenergy per unit area and r is the radius of the domain. The force actingto change the domain size is given by or 41r7'o',,, This can be comparedto the force produced by a conventionally applied bias field H or 41rThMH where h is the layer 22 thickness and M is the saturation magneticmoment of layer 22 material. If we normalize each of forces (2) and (3)in terms of an applied bias field H we recognize a new effective fieldwhich we can designate as H Hr: M4,

At this thickness, shown in FIG. 4, the cylindrical domains are stableand will remain stable without the necessity of an external appliedfield.

A convenient structure of the type shown in FIG. 2 is prepared, forexample, by suitably polishing a sheet comprising a crystal of terbiumorthoferrite (TbFeO plus bismuth (one percent). Typically, the sheet hasa thickness of 1.7 mils and each surface has a strain layer of onemicron thickness to provide a coercivity there for enabling amagnetization condition which is only negligibly changed duringoperation. Single wall domains having diameters of 2.0 mils have beenmoved in such a sheet by propagation fields of about 10 oerstedsgenerated as indicated in FIG. 1.

There are alternative structures wherein additional layers are depositedon the surfaces of a suitable sheet of material in which single walldomains can be moved. For example, high coercivity layers ofmagnetoplumbite can be deposited on terbium orthoferrite to this end.All that is necessary is that the surface layers, whether grown ordeposited, be of sufficiently high coercivity to remain set permanentlyduring operation and that the layers be exchange-coupled to the bodylayer.

What has been described is considered only illustrative of theprinciples of this invention. Consequently, various modifications inaccordance with those principles can be devised by one skilled in theart without departing from the spirit and scope of this invention.

What is claimed is:

1. A domain propagation arrangement comprising a sheet of magneticmaterial in which single wall domains can be moved, said sheet beingcharacterized by a preferred direction of magnetization along an axisout of the plane of said sheet and having first and second surfacelayers and a body layer, each of said surface layers being effectivelypermanently set in a direction along said axis and exchange-coupled tosaid body layer, and means for moving single wall domains in said bodylayer in a manner to avoid changing the magnetization in said surfacelayer.

2. An arrangement in accordance with claim 1 wherein said axis issubstantially normal to the plane of said sheet.

3. An arrangement in accordance with claim 2 wherein said sheetcomprises a crystal of a rare earth orthoferrite including about onepercent bismuth.

4. An arrangement in accordance with claim 3 wherein said sheetcomprises a crystal of terbium orthoferrite having strained surfacelayers for maintaining a preset condition of magnetization therein.

5. An arrangement in accordance with claim 2 wherein said body layercomprises a rare earth orthoferrite and said surface layers compriserelatively high coercivity layers of magnetoplumbite.

6. An arrangement in accordance with claim 5 wherein said body layercomprises terbium orthoferrite.

References Cited UNITED STATES PATENTS 3,092,813 '6/1963 Broadbent340174 JAMES W. MOFFITT, Primary Examiner

